MECHANISM OF INFECTION AND IMMUNITY 225 



to study some of the phenomena of infection. In doing this we will 

 confine ourselves to bacteria. 



Bacteria. There are some widely prevalent views concerning bac- 

 teria which in my opinion are quite erroneous. It is generally stated 

 that bacteria are low forms of plant life. This belief is founded on 

 an early observation that they are not readily soluble in dilute acids 

 or alkalies. Is this enough to justify their classification as plants? Hair, 

 skin and horn are not readily soluble in dilute acid or alkali and still 

 they can hardly be called plants. Plant cells, generally, at least, con- 

 tain cellulose; bacteria do not. Plants, under normal conditions, take 

 in carbonic acid and give off oxygen ; bacteria absorb oxygen and give 

 off carbonic acid. Many think that bacteria contain no nuclei, because 

 there is no differentiation in staining, but it should be remembered 

 that their staining properties show that they are practically wholly 

 composed of nuclein. Some think that they are of simple chemical 

 structure, because morphologically they are simple. I and my students 

 have shown that, chemically, bacteria are quite as complicated and as 

 highly developed as are the cells of man's body. Functionally, they 

 are highly developed. It is important to hold this in mind in studying 

 the contests between bacterial and body cells, which so often end 

 in the discomfiture of the latter. 



Bacteria live and multiply through the activity of their enzymes. 

 Their extracellular enzymes split the pabulum within their reach into 

 proper blocks and their intracellular enzymes fit these blocks into the 

 bacterial molecule. It must be plain that a bacterium, whose enzymes 

 cannot act on body proteins, cannot infect that animal. Such a 

 bacterium may grow outside the animal body, feed on dead material 

 and elaborate a poison which may harm the animal. Such a bacterium 

 is the Bacillus botulinus. The peptonizing bacteria of milk so change 

 the milk proteins that they are absorbed through the intestinal walls 

 of infants and are further digested in the blood and tissues with the 

 formation of poisons which cause the symptoms and lesions of cholera 

 infantum and the other diarrheal diseases of infancy. During intra- 

 uterine life, all the processes of digestion are parenteral, that is, they 

 do not occur in the intestine but in the blood and tissues. In infancy, 

 the walls of the intestine are easily permeable and parenteral digestion 



